1. Field of the Invention
This invention relates to a focal point detecting apparatus for detecting a focusing state of an optical system for use in, for example, cameras or the like, and more particularly to a facal point detecting optical apparatus for the use of multi range finding work in which a focusing state is detected at several places on an optical axis of a taking lens and outside the optical axis.
2. Description of the Prior Art
Conventional focal point detecting optical apparatus for multi range finding work includes range finding zones which are spaced apart so that beams of light of focal point detecting optical systems will now be interfered with each other. Therefore, images which are reform for detecting a focal point are spaced apart and light receiving elements (e.g., CCD line sensors) are impossible to be formed in positions proximate to each other.
Therefore, in case light receiving elements are located on one chip, a substrate becomes large. On the contrary, in case elements are located on independent substrates, verification of alignment of optical axes, etc. must be performed per each substrate. In any case, the space occupied by the sensor portion becomes large. As a result, AF (Auto Focus) module becomes large as a whole and costs are increased.
In view of the above, inventors of the present invention propose a constitution, in which prisms 4,5, as shown in FIG. 8, are disposed behind condenser lenses 1,3 adapted to receive beam of light outside an optical axis l of a taking lens (not shown) so that a reimaging position will approach to the optical axis l (japanese patent application No. Sho. 63-64836).
Beam of light made incident from the taking lens is defined in three zones (range finding zones) by a field mask 6 which is provided in such a manner as to be coincident with an imaging surface, and a pair of images of an object to be photographed are reimaged on each CCD sensor 9,10,11 through condenser lenses 1,2,3, prisms 4,5, a diaphragm mask 7, and a separator lens 8.
According to this constitution, the beam of light coming from the range finding zone outside the optical axis l of the taking lens is bent toward the optical axis l side by means of the function of the prism 4,5 and by means of prism function due to eccentricity of the condenser lenses 1,3. Therefore, the reimaging positions can be approached to each other, and the substrate 12 provided with the CCD line sensors 9,10,11 as well as the whole AF module can be made small.
However, with this constitution, since the deflecting angle of an optical path for a focal point detecting optical system located outside the optical axis l is comparatively large, when the substrate 12 is adjusted its position toward the optical axis direction, the CCD line sensors 9,10,11 deviate from the position covering the range finding zones by slight movement.
Also, in order to make the thickness of the prism 4 smaller than a predetermined value, the deflecting angle cannot be produced only by the function of the prism 4. Therefore, the prism function due to the eccentricity of the condenser lens 1 is also utilized as mentioned above. Therefore, a passing zone PZ of the beam of light, as shown in FIG. 9, becomes the peripheral portion of the condenser lens 1. In order to obtain this zone PZ, the outer diameter of the lens and the central thickness thereof become large and the generation of aberration is large, too.
Furthermore, since the space between the imaging surface and the prism 4 is large, there arises such a problem as that lateral chromatic aberration is large.